Method of forming a zinc-aluminum coating on a ferrous base



United States Patent 3,325,282 METHOD OF FORMING A ZINC-ALUMINUM CQATING ON A FERROUS BASE Edward H. Mayer and Hilton N. Rahn, Bethlehem, Pa.,

assignors to Bethlehem Steel Corporation, a corporation of Delaware No Drawing. Filed Apr. 27, 1965, Ser. No. 451,328 11 Claims. ((1 75208) This application is a continuation-in-part of application Ser. No. 382,659 filed July 14, 1964, and now abandoned.

This invention relates to the production of zinc-aluminum coatings on ferrous surfaces, and more particularly to the forming of such coatings 'by a sintered powder method.

In the protection of ferrous surfaces from corrosive media, it is common practice to coat the ferrous base metal with a non-ferrous metal by immersion in .a molten bath of the coating metal. Among the coating metals commonly used are zinc and aluminum. Non-ferrous coatings, of the type described, must not only be protective; for most commercial applications they should also be ductile, tightly adherent, and metallurgically bonded to the base. It is also desirable that these coatings have a smooth, pleasing appearance, and that the iron-bearing alloy layer between the ferrous surface and the coating be relatively thin and uniform.

Although generally zinc or'aluminum has been used individually as a coating metal, efforts have been made to combine the two metals for the purpose of obtaining a coated product which exhibits properties superior to those obtained with zinc alone, and, in certain instances, when aluminum is used alone. However, it has heretofore been diflicult to produce ferrous products having zinc coatings containing substantial amounts of aluminum, which coatings are corrosion resistant, tightly adherent, metallurgically bonded to the ferrous base, and free from substantial amounts of iron-bearing alloy at the interface betwee the base and the coating. p

It is a principal object'of this invention to provide a method by which a satisfactory pine-aluminum coating can be applied to a ferrous base.

Another object is to provide a method of producing a z nc-aluminum coating which is metallurgically bonded to a ferrous base. V

A further object is to provide a method of producing a Zinc-aluminum coating on a ferrous base which coating is ductile and firmly adherent.

We have discovered that the foregoing objects can be attained by applying to a ferrous base having a roughened surface, a powder consisting essentially of 5% to 95% aluminum, with the balance zinc, compacting the powder on said base, and heating the compacted powder and base for a time and at a temperature sufiicient to sinter said powder and form a metallurgical bond between said powder and said base. Our method, which may be referred to as roll bonding, is applicable to any flat ferrous base,

the surface of which has been roughened to provide a surface receptive to the powder which is subsequently applied thereto. Various methods may be used to roughen the surface, for example, lightly etching the base inacidor abrading the surface with emery cloth. Where the ferrous base is hot-rolled steel strip, the normal pickling treatment following hot rolling satisfactorily roughens the strip surface.

3,325,282 Patented June 13, we?

Briefly, our method comprises applying to the roughened surface of the ferrous base a substance which provides a temporary bond between the base and the powder which is subsequently applied thereto. This substance must be sufiiciently volatile so that it is completely removed from the coated base during the heating step of our method. One substance having these required properties is tridecyl alcohol.

4 The coating powder is then uniformly applied to the surface of the ferrous base. Said powder consists essentially of 5% to 95% aluminum, balance Zinc.

The powder is next compacted and mechanically bonded to the surface of the ferrous base by subjecting the powder-coated base to compressive forces. Said forces may be applied by means of rolls.

The ferrous base, with the compacted powder mechanically bonded thereto, is then heated to an elevated temperature, and held at said temperature for a time sufficient to sinter the powder and form a metallurgical bond between said powder and said base.

In order to obtain satisfactory coatings by the method of this invention, it is necessary to control the time and temperature during the sintering operation. While the temperature may range, broadly, from about 600 F. to 850 F., within this broad range there will be a considerable variation in the time required for sintering. At the lower end of the temperature range, long sintering periods of about 6 hours minimum will be required, the time requirement gradually decreasing as the sintering temperature' increases, until/at the high end of the range, the period required for proper sintering may be less than one minute.

Control of heating times is dictated by the coating characteristics required. When heating times of less than about 6 hours are used, at a temperature in the neighborhood of 600 F., the coating may have poor adherence,

indicating insufficient metallurgical bonding at the coating-base metal interface, and/or lack of coherence be: tween individual coating particles. Although 6 hours sin-V tering time is generally adequate for temperatures ranging from 600 F. to 675 F., or those temperatures in which a reasonably long heating period does not result in exces-v sive alloying at the coating interface, longer times, up to. 24 to 48 hours may be preferable. Longer sintering times assure a uniform thermal treatment throughout the large coils of material treated in commercial production. On the other hand, at the higher sintering temperatures, the duration of the heating period must be controlled to pre vent diffusion of iron into the coating to the extent that brittle iron-zinc compounds are formed at the surface of the coating. Excessive formation of intermetallic compounds of iron with zinc produces a brittle coating which will withstand little or no deformation. Furthermore, when iron-zinc compounds develop to the point where they penetrate the coating surface, the corrosion resistance of the coating is lowered appreciably. To prevent The following specific examples are given in further illustration of the invention. Parts or percentages shown,

unless otherwise specified, are on a weight basis.

Example I A 20-gage, cold-rolled annealed steel sheet containing 0.003% carbon was etched for 5 minutes in a 6.5 weight 3 percent sulfuric acid solution, and then filmed with tridecyl alcohol. The alcohol was applied by wiping the sheet with a cloth saturated with the alcohol. The metal powder, from which the coating was subsequently formed, was a preblended mixture of 25% aluminum and 75% zinc. The aluminum powder, which had an average particle diameter of 26 microns, had a chemical analysis of:

1 Approximately.

The zinc powder had an average particle size less than 20 microns in diameter, and a chemical analysis as follows:

Percent Zinc 1 99.0 Aluminum 0.30

Cadmium Co per Iroii 0.08 Lead 0.11 Magnesium 0.01 Nickel 0.06 Silicon 0.015

Total bismuth, chromium, gallium, germanium,

indium and tin 0.01

1 Approximately. The powder was applied to the filmed steel sheet by passing the sheet through a bed of the powder mixture, and removing excess powder by lightly tapping the sheet. The sheet, with powder uniformly distributed on both sides in an amount of approximately 11 grams per square foot, was passed through a rolling mill having 4-inch diameter work rolls, to compact the powder onto the sheet, as well as to compact individual adjacent powder particles. During compacting, the sheet was elongated approximately 3%. Heating of the sheet with the compacted powder thereon was performed in a resistance furnace in an atmosphere of 100% hydrogen. The furnace was first brought to a temperature of 750 F., and the sheet then introduced into the furnace. The sheet was brought to the 750 F. temperature and held there for 5 minutes, after which the sheet was cooled to ambient temperature in the protective atmosphere. The coated sheet was given a skin pass in the rolling mill to improve surface brightness of the coating. The resultant coating was both ductile and adherent, and metallurgically bonded to the steel base.

Example II An 18 gage, hot rolled, mill pickled steel strip, containing 0.08% carbon, was solvent cleaned, then filmed, powder coated and the coated product compacted as in the previous example. In this example the powder was composed .of 25% aluminumand 75% zinc and the sheet and compacted powder were held at a sintering temperature of 810 F. for less than 1 minute in an atmosphere consisting of 18% hydrogen and 82% nitrogen.

Example III comprised 50% aluminum and"50% zinc, and washeld 4 at a sintering. temperature of 600 F. for 24 hours in an 18% hydrogen, 82% nitrogen atmosphere.

Example IV A strip of the same material as that of Example II was solvent cleaned and the powder applied as in the previous examples. The powder comprised 50% aluminum and 50% zinc, and the coated product was heated at a sintering temperature of 810 F. for less than 1 minute in an 18% hydrogen, 82% nitrogen atmosphere.

Although heating, in Examples I-IV, was performed in an atmosphere of either pure hydrogen or a mixture of hydrogen and nitrogen, a protective atmosphere is not essential for the production of a ductile, adherent corrosion resistant coating when both sides of the sheet are coated. However, if it is desired to coat only one side of the sheet, a protective atmosphere is desirable. A mixture of hydrogen and nitrogen in any combination wherein the hydrogen is present in an amount not less than about 4%, will prove adequate to protect the exposed steel surface.

When both sides of the strip are coated, it is usually preferable to sinter in air or other oxidizing atmosphere. An oxidizing atmosphere has been found to have a restraining efiect on the diffusion of iron from the base metal into the zinc phase of the coating. Through this restraining action of air, it is easier to control the sintering rate at the higher temperatures, or in the longer soaking times at lower temperatures. When diffusion proceeds at a rapid rate, there is danger of producing too much interfacial alloy, which may render the coating non-adherent.

In each of the examples from V to XII, given below, sintering was performed in air.

Example V Zinc-aluminum powder containing 20% aluminum and zinc. was applied to a base metal strip as in Example II, and the resultant compacted powder and strip sintered in air at 600 F. for 48 hours.

Example VI In this example the base steel, the preparation thereof, and the application of the powder were the same as in Example -I. The powder contained 35% aluminum and 65% zinc, and the compacted powder and strip were sintered in air at760 F. for 5 minutes.

Example VII A strip of the type used in Example II was prepared and powder coated and the powder compacted as in that example. The powder contained 35% aluminum and 65% zinc and the resultant compacted powder and strip were sintered in air at 850 F. for less than 1 minute.

Example VIII A strip of the type used in Example II was prepared and coated as in that example. The powder contained 54% aluminum and 46% zinc, and the compacted powder and strip were sintered in air at 825 F. for less than 1 minute.

Example IX A powder containing 35% aluminum and 65% zinc was applied to a base strip of the type used in Example II, and compacted as in that example. The compacted powder and strip were sintered in air at 650 F. [for 24 hours.

Example X In this example the powder comprised 54% aluminum and 46% zinc. The powder was applied to a base strip of the type used in Example II, and compacted as previously described, and the resultant product sintered in air at 675 F. for 24 hours.

Example XI A powder containing 75% aluminum and 25% zinc was applied to a base strip of the type used in Example II, and compacted as in the previous examples. The resultant product was sintered in air at 650 F. for 24 hours.

Example XII Powder containing 35% aluminum and 65% zinc was applied to a 20 gage cold rolled annealed base strip containing 0.06% carbon. Preparation of the strip, application of the powder, and compacting were performed as in Example I. The compacted powder and strip were sintered in air at 825 F. for less than 1 minute.

In order to determine coating adherence with different proportions of zinc and aluminum, a series of coated sheets was made, by the method of Example 1, covering a range of aluminum of from 5% to 95%. Sheets were produced having coatings containing respectively, 5%, 35%, 50%, 75% and 95% aluminum. Each of these sheets was heated at 750 F. for 5 minutes. In each case zinc made up the balance of the coating. In this series, each sheet showed excellent coating adherence.

For the purpose of evaluating corrosion resistance properties of the coated article made by our method, a series of coated sheets representative of each of the examples given above, was given an Olsen cut test. The sheets displayed no noticeable cracking or flaking at a cup depth of 0.2 inch. The method for performing the Olsen test is described in The Making, Shaping and Treating of Steel, 7th Ed. (1957) at pages 923-924. Following the Olsen test, each specimen was subjected to a salt spray test. The test was conducted in accordance with A.S.T.M. specification 13117-62. The tested sheets had a coating thickness on each side of approximately one mil. Results from this test are shown in the table below:

Sample from First rusting Example: hours I Between 2375-2445 II Between 1945-2035 III No rust at 1 5620 IV Between 3465-3555 V Between 1305-1395 VI Between 1685-1755 VII Between 2425-2515 VIII Between 1630-1700 IX Between 3495-3585 X Between 1875-1945 XI Between 2800-2870 XII Between 3130-3225 1 Test terminated.

Samples of commercially galvanized sheet showed first rusting at between 415 and 485 hours.

Coated sheets, similar to those used in the salt spray test, were bent through a 130 are over their own thickness without any noticeable flaking or cracking.

In the high temperature, short time treatments, it is preferable to quench the sintered product after heating to prevent the possibility of further interfacial alloy growth.

While the base stock used in the examples was made from low carbon sheet steels, the invention is not limited to these steels.

Other coated sheets produced by our method and which had a coating composition of 25% aluminum, 75 zinc, were bent through 180 over their own thickness and immersed in tap water at room temperature. These sheets showed no sign of failure after 5 months of immersion in tap water.

Although the examples of our method refer to coating sheets, it will be readily observed that the method is one which lends itself to the coating of any flat ferrous base, e.g., steelstrip. When strip is coated, the steps of filming, applying powder and compacting can be made continuous. In a continuous process it may be desirable to apply the powder to the base metal by means of a fluidized bed. If times of from 30 to 60 minutes or more are necessary for the heating step, this step would probably best be performed by placing the strip in the heating furnace in coil form. For shorter heating times, the strip could be cut into sheets, and the sheets placed in the furnace in spaced groups. On the other hand, a continuous method of heating the strip may be preferable for the shorter heating periods.

While the coatings produced by the method of the invention consist essentially of the metals zinc and aluminum, other substances which do not materially detract from the novel and basic features of our invention may be present, either as impurities or as deliberate additions. By the term consisting essentially of we do not wish to exclude the presence of such substances.

We claim:

1. The method of forming a non-ferrous coating on a ferrous base having a roughened surface comprising applying to the base a powder consisting essentially of from 5% to aluminum by weight, the balance zinc, compacting the powder on said base and heating the compacted powder and base for a time and at a temperature suificient to cause diffusion between the compacted powder and the base which will produce an adherent coating but insuflicient to cause substantially all of the zinc to combine with iron as an intermetallic compound.

2. The method of forming a non-ferrous coating on a ferrous base having a roughened surface comprising applying to the base a powder consisting essentially of from 5% to 95 aluminum by weight, the balance zinc, compacting the powder on said base, and heating the compacted powder and base within the temperature range of 600 to 850 F. for a time sufficient to cause diffusion between the compacted powder and the base which will produce an adherent coating but insufficient to cause substantially all of the zinc to combine with iron as an intermetallic compound.

3. A method according to claim 2 wherein the temperature range is from 600675 F.

4. A method according to claim 2 wherein the temperature range is from 700-775 F.

5. A method according to claim 2 wherein the temperature range is from 800-850 F.

6. A method according to claim 2 wherein the compacted powder and the base are heated in an oxidizing atmosphere.

7. A method according to claim 2 wherein the compacted powder and the base are heated in a non-oxidizing atmosphere.

8. The method of forming a non-ferrous coating on a ferrous base having a roughened surface, comprising applying to the base a powder consisting essentially of 5% to 95 aluminum by weight, the balance zinc, compacting the powder on said base, and heating the compacted powder and base within the temperature range 600 F. to 675 F. for 6 to 48 hours.

9. The method of forming a non-ferrous coating on a ferrous base having a roughened surface, comprising applying to the base a powder consisting essentially of 5% to 95 aluminum by weight, the balance zinc, compacting the powder on said base, and heating the compacted powder and base within the temperature range 725 F. to 760 F. for 3 to 30 minutes.

10. The method of forming a non-ferrous coating on a ferrous base having a roughened surface, comprising applying'to the base a powder consisting essentially of References Cited UNITED STATES PATENTS 2,848,797 8/1958 Eubank 117131 X 3,056,694 10/1962 Mehler et a1 117131 X 5 7 3,088,195 5/1963 Noethlich et a1. 75-208 X 3,104,135 9/1963 Morrison et a1. 75208 X CARL D. QUARFORTH, Primary Examiner.

BENJAMIN R. PADGETT, Examiner.

M. J. SCOLNICK, Assistant Examiner. 

1. THE METHOD OF FORMING A NON-FERROUS COATING ON A FERROUS BASE HAVING A ROUGHENED SURFACE COMPRISING APPLYING TO THE BASE A POWDER CONSISTING ESSENTIALLY OF FROM 5% TO 95% ALUMINUM BY WEIGHT, THE BALANCE ZINC, COMPACTING THE POWDER ON SAID BASE AND HEATING THE COMPACTED POWDER AND BASE FOR A TIME AND AT A TEMPERATURE SUFFICIENT TO CAUSE DIFFUSION BETWEEN THE COMPACTED POWDER AND THE BASE WHICH WILL PRODUCE AN ADHERENT COATING BUT INSUFFICIENT TO CAUSE SUBSTANTIALLY ALL OF THE ZINC TO COMBINE WITH IRON AS AN INTERMETALLIC COMPOUND. 